One's daily physical activity level is increasingly identified as an important contributor to and indicator of general health, risk of heart disease and musculoskeletal maintenance. For example, recent investigations indicate that declining activity level with age is a likely contributor to decreased muscle mass and strength, and decreased regional bone density in the elderly. However, measures of daily activity that may apply to the health and maintenance of the cardiovascular system may not apply to the musculoskeletal system. While the two are closely linked by the type and quantity of physical activity, cardiovascular fitness is assessed by metabolic measures whereas musculoskeletal fitness is quantified in terms of bone density and muscle strength and endurance.
The traditional approach to quantifying daily activity level has been to estimate the rate of daily energy expenditure in kilocalories per day from logbooks, pedometers, or activity monitors that record limb or trunk motion or accelerations. These methods provide some quantitative metabolic information about a subject's habitual activity level, but give no indication of the magnitude and frequency of musculoskeletal loading, particularly at relevant skeletal sites.
It is generally acknowledged and numerous studies concur that individuals engaged in activities generating high musculoskeletal forces have higher muscle mass and bone mass or density in regions subjected to the higher levels of force. Theoretical models and experimental evidence suggest that functions of the average daily history of peak cyclic tissue stresses and strains, when properly analyzed, will correlate to important functional measures of musculoskeletal fitness. While these models have not incorporated the effect of load-rate on bone or muscle adaptation explicitly, a number of experimental studies have indicated that load-rate may be an important factor contributing to an osteogenic stimulus. It has also been suggested that high load-rates accelerate the aging of cartilage tissue leading to osteoarthritis.
It is not possible to assess the contribution to musculoskeletal maintenance from daily activity including exercise without a method of monitoring and quantifying the significant external forces loading the body during normal daily activity. By virtue of force and moment equilibrium, high external ground reaction forces (GRFs) and ground reaction force-rates (GRF-rates or load-rates) are coincident with high lower limb internal muscle and bone forces, stresses/strains and stress-/strain-rates. Of the three components of the resultant ground reaction force, the vertical component, GRFz, is the most significant component loading the human body during normal daily activities. Studies have shown that the energy cost of walking is related to body weight and walking speed. Recent studies have also shown that the metabolic cost of locomotion is directly proportional to the magnitude of GRFz and duration of running.
It follows that a device that monitors, records and processes long term peak cyclic GRF and GRF-rate histories, and in particular, peak GRFzs and GRFz-rates, would be a unique and useful means of assessing normal daily activity in terms of metabolic and musculoskeletal variables. We have previously reported on a GRFz recording system that uses an analog capacitance insole sensor connected to an analog-to-digital converter and microcontroller worn at the waist (Whalen et al., 1993). This system has demonstrated the feasibility of collecting GRFzs, but is expensive, requires frequent calibration and replacement of the insole, and is not easily miniaturized.
A number of issued patents describe instrumented shoes or insoles for evaluation of exercise sessions. Yukawa (U.S. Pat. No. 4,649,552) describes a step counting device which is attached to an existing shoe, and measures distance traveled based on a fixed estimate of average stride length. Dassler (U.S. Pat. No. 4,703,445) describes a device which accounts for step-to-step gait variability during running by means of ultrasonic range finding between the left and right shoes.
Frederick (U.S. Pat. No. 4,578,769) and Cavanagh (U.S. Pat. No. 4,771.394), describe devices which track temporal characteristics of ground contact events to improve estimation of exercise-related parameters during running such as speed, distance traveled and energy (calories) expended. Similarly, Kato (U.S. Pat. No. 5,033,013) describes a device for determination, based on regressions to footfall frequency, of speed, distance traveled, and energy expenditure during walking. Each of these devices is designed to estimate work performed during either walking or running activity, but not both.
Devices based on step counting alone do not consider the magnitude of musculoskeletal loading, and alone cannot account for individual gait characteristics, gait speed and unpredictable high loading events that may influence bone and muscle significantly. For example, these devices cannot distinguish between slow and fast walking, both of which occur commonly during daily activity and differ significantly with respect to the magnitude of associated peak musculoskeletal forces. However, devices which consider the temporal characteristics of the footfall pattern may account for some of these variations.
Frederick (U.S. Pat. No. 4,578,769) observed an approximately linear relationship between running speed and foot-ground contact time, and subsequently describes a device which measures contact time to account for step-to-step gait variability to improve prediction of total distance from step count during running. Furlong (U.S. Pat. No. 4,956,628) describes a device which uses pressure sensors or contact transducers to detect the presence or absence of contact between both feet and the ground, but makes no interpretation of the duration or frequency of these events.
A number of devices have been disclosed which measure physical variables during human activity. Sidorenko et al. (U.S. Pat. No. 4,409,992) describe an electronic ergometer worn at the waist to measure the amount of performed work and magnitude of power developed by an individual during walking or running. Bianco (U.S. Pat. No. 4,855,942) describes a step counter worn on the wrist. However, activity monitors placed on the arm or waist are unreliable indicators of musculoskeletal loading. Another device described by Sidorenko et al. (U.S. Pat. No. 4,394,865) consists of an instrumented seismic mass connected to an alarm system for detection of abnormally high loads during exercise. This device is intended as an alarm system for overexertion, and is not designed as an ergometer. Carlin (U.S. Pat. No. 4,774,679) describes a device which can be attached to the ankle to measure and record local accelerations to estimate impact forces during athletic activity in humans and animals. Wood (U.S. Pat. No. 5,373,651) describes a device which estimates and stores force applied to an athletic shoe from a plurality of pressure sensors imbedded in the shoe sole.
None of the existing art describes devices to assess cumulative musculoskeletal loading during normal daily activity, which can consist of a combination of sedentary activity, walking, and running, in various proportions depending on the individual. None of the existing art can distinguish between and account for the differences between walking and running. The magnitude of the GRF, and consequently internal musculoskeletal loads, varies greatly with gait speed for both walking and running. Prior to the present invention, temporal parameters of gait have not been used to predict peak GRF. Although this can be tracked by direct measurement of the GRF, the temporal approach may be advantageous because temporal measurements can be made more easily, cheaply, and reliably than direct force measurements.